Several studies in animals and humans have shown a decline in the rate of drug metabolism with aging. The mechanisms involved include decreased microsomal enzyme activity and enzyme inducibility, decreased liver mass, and decreased hepatic blood flow. The influence of cofactor supply in determining the rate of drug metabolism in aged subjects has never been evaluated. Recent work by the applicant has shown that, depending upon the nutritional state, mitochondrial oxidative pathways supply a majority of the reducing equivalents for mixed-function oxidation. For example, up to 75% of the reducing equivalents are provided in the fasted state by Beta-oxidation of fatty acids. Thus a project is proposed to test the hypothesis that age-dependent changes in mitochondrial function may cause a decrease in the rate of Beta-oxidation of fatty acids and hence a decrease in the rate of mixed-function oxidation in the perfused liver. Preliminary work showing that mixed-function oxidation may alter rates of ketogenesis will also be further explored. Because mitochondria are most highly concentrated in the periportal region of the liver lobule, studies using micro-light guides to detect surface fluorescence of NADH will be used to determine whether this distribution causes parallel inhomogeneity in the distribution of fatty acid oxidation and drug metabolism across the liver lobule. Finally, comparisons of rates of Beta-oxidation and drug metabolism will be made in perfused livers of 8 adult and aged inbred strains of mice to determine whether any age-related changes in mitochondrial or microsomal function are genetically determined.